When manufacturing a tire, a green tire formed of unvulcanized rubber is vulcanized for a predetermined period of time. Generally, the vulcanization time is determined taking a blow point time of the unvulcanized rubber (the rubber composition) as reference. The blow point time is a minimum time required for air bubbles to no longer be generated in the interior portion of the rubber composition when the rubber composition that is being vulcanized under pressure is removed from the molding die and is returned to atmospheric pressure in order to end the vulcanization step. Various variations occur in the molding step and the vulcanization step, and these variations are therefore taken into account in the vulcanization time. Taking a degree of equivalent vulcanization that corresponds to a blow point time of the unvulcanized rubber in the vulcanization rate-limiting section of the tire as reference, a blow point time of the tire required to reach the vulcanization time is established, a predetermined safety time is added and the vulcanization time is set to prevent insufficient vulcanization. On the other hand, if the vulcanization time becomes longer, productivity deteriorates and there may also be an adverse impact on the physical properties of the rubber due to over-vulcanization. There is therefore a demand to shorten the vulcanization time as much as possible, while preventing insufficient vulcanization.
For example, a control system has been proposed in which an optimum vulcanization time is obtained in advance for each individual tire, and control is performed to carry out vulcanization for this optimum vulcanization time, thus eliminating a deterioration in tire performance due to over-vulcanization, and uniform tires are obtained with a high level of efficiency (see Japanese Unexamined Patent Application Publication No. 2007-98756A). Specifically, in the control system described in Japanese Unexamined Patent Application Publication No. 2007-98756A, a reference vulcanization time is calculated in advance on the basis of reference values of a plurality of characteristic values that affect the vulcanization time. Then, for each individual tire to be vulcanized, measurement results obtained by measuring each of the characteristic values are compared with the respective reference values and a difference therebetween is calculated. A reference vulcanization time is corrected using a vulcanization time corresponding to the difference, and thus the vulcanization time when the vulcanization is actually carried out is calculated. However, in order to perform the correction and calculate the vulcanization time, it is necessary to ascertain, through experiments and FEM calculations, a degree of impact of each of the characteristic values on the vulcanization time. What is more, as it is necessary to ascertain the degree of impact for each different tire specification, a great amount of operation man-hours is required, and it is necessary to create a large scale database.
Further, in order to ascertain a state of heat conduction in the tire vulcanization step, a three-dimensional thermal conduction/vulcanization reaction rate analysis model is also used. However, a high level of technology and a great amount of time are required to create the three-dimensional analysis model. Further, as a great amount of time is required to perform the calculations at the time of analysis, there is a problem that is it not possible to instantly obtain the optimum vulcanization time.